part: Allocate only one legacy_mbr buffer

Commit ff98cb90

 ("part: extract MBR signature from partitions")
blindly switched allocated by ALLOC_CACHE_ALIGN_BUFFER buffer type from
"unsigned char" to "legacy_mbr" which caused allocation of size =
(typeof(legacy_mbr) * dev_desc->blksize) instead of just space enough
for "legacy_mbr" structure.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Tom Rini <trini@konsulko.com>